The invention relates to an optically diffractive security element, as set forth in the classifying portion of claim 1.
Such security elements are used for the authentication of documents such as value-bearing papers or bonds, cheques, banknotes, credit cards, passes and identity cards of all kinds, entrance tickets, driving licences and so forth, the security element being for example in the form of a thin layer composite or laminate, which is fixed on the document by adhesive.
Modern copiers for coloured copies represent a serious potential danger for documents which are produced by a printing procedure, because the visual differences between the original and the copy are so slight that only an expert equipped with the appropriate aids can distinguish the original from the copy, in which respect it is often necessary to refer to other criteria such as intaglio printing, a watermark, fluorescence, optically variable security elements with diffraction structures and so forth, apart from the appearance of the printed image.
It is known from EP 0 522 217 B1 that reflective foil portions arranged on a document implement good protection against unauthorised copying of such documents. The difference between the original with the reflective foil portions and a copy can be clearly seen as the copying machines reproduce reflective surfaces as black. It will be appreciated however that reflective foils are readily available on the market. The black surfaces in such copies can therefore easily have reflective foil stuck over them, in order to make the copy appear more genuine.
DE 44 10 431 A1 describes further developments of the above-described foil portions. The security element is a foil portion which is cut from a laminate, with a flat, mirroring reflection layer. The reflection layer is removed in surface portions which form an individual identification on the surface of the foil portion, so that a black layer arranged under the reflection layer becomes visible. On the copy produced by the copier machine, the black identification disappears in the reproduction of the remaining mirror surface, as the surface portions in which the reflection layer is removed and the mirror surface which has remained behind in the copy appear uniformly black. Another security element, instead of the flat mirror surfaces, has a hologram structure with the identification and, in the copying procedure, behaves like the diffraction structures which are discussed in the next paragraph. In the copy therefore, the identification can be detected in the copied image of the hologram.
It is also known for example from GB 2 129 739 B for valuable documents to be provided with an optically variable security element having diffraction structures (for example holograms, mosaic-like surface patterns comprising diffractive surface elements, for example in accordance with EP 0 105 099 A1, EP 0 330 738 A1, EP 0 375 833 A1, and so forth). Those security elements have a pattern or image which changes in dependence on the viewing condition. From the point of view of unauthorised persons, those security elements can be imitated only at high cost. Unfortunately, the colour copy of the document reproduces one of the patterns or images of the security element, which is visible in the original under the viewing condition which is fixed in the copier for the imaging procedure. It will be appreciated that it is no longer possible to see any change in the pattern or image in the copy, upon a variation in the viewing condition, but if the receiver is not paying attention, a copy can easily be considered to be the genuine document.
Embodiments of the laminate for the security elements and materials which can be used for that purpose are described in EP 0 401 46 A1 and U.S. Pat. No. 4,856,857.